Scheduled to launch June 27th, the IRIS spacecraft will point a telescope at the interface region of the sun that lies between the surface and the million degree outer atmosphere called the corona. It will improve our understanding of how energy moves from the sun’s surface to the glowing corona, heating up from 6,000 degrees to millions of degrees.

Engineers work with the IRIS spacecraft on the nose of the Pegasus XL rocket that will launch the solar observatory into Earth orbit. (Photo credit: VAFB/Rnady Beaudoin)

The IRIS mission, short for Interface Region Imaging Spectrograph, calls for the 7-foot-long spacecraft to point its ultraviolet telescope at the sun to discern features as small as 150 miles across. It will look at about 1 percent of the sun’s surface.

“IRIS will show the solar chromosphere in more detail than has ever been observed before,” said Adrian Daw, deputy project scientist. “My opinion is that we are bound to see something we didn’t expect to see.”

IRIS is a NASA Small Explorer that will complement the Solar Dynamics Observatory and Hinode missions to explore how the solar atmosphere works and impacts Earth. SDO and Hinode will monitor the solar surface and outer atmosphere, with IRIS watching the region in between.

“IRIS almost acts as a microscope to SDO’s telescope,” said Jim Hall, mission manager for IRIS. “It’s going to look in closely and it’s going to look at that specific region to see how the changes in matter and energy occur in this region. It’s going to collectively bring us a more complete view of the sun.”

IRIS improves our understanding of the interface region where most of the sun’s ultraviolet emission is generated that impacts the near Earth space environment and Earth’s climate. Solar activity such as coronal mass ejections and solar flares, also are of great interest to spacecraft designers who have to figure out ways to protect instruments and electronics from them.

“We’re always looking for the answers to why and everything starts at the root with the sun,” Hall said.

IRIS will ride into Earth orbit on an Orbital Sciences Pegasus XL rocket. The Pegasus is famous as the only winged launcher in NASA’s inventory. Though small compared to the gigantic boosters that send heavy satellites into orbit and probes to distant worlds, the Pegasus’ size and flexibility has allowed numerous missions to be launched that would have been too small for larger rockets.

“Pegasus has been a tremendously successful launch vehicle for NASA,” said Tim Dunn, launch director for IRIS. “We have launched 18 successful missions on Pegasus. The team is very dynamic, very flexible. They’re able to accomplish a tremendous amount in a very short time.”

The Pegasus and its IRIS payload will be carried to about 39,000 feet under a modified L-1011 airliner taking off from Vandenberg Air Force Base in California. Over the Pacific Ocean off the California coast, the plane will drop the Pegasus to begin the launch.

The Pegasus will ignite its solid-fueled first stage five seconds into its fall and arch skyward with the main wing giving it lift and the three fins in the back steering it through the thick layers of Earth’s lower atmosphere.

The rocket will burn its load of fuel in 73 seconds and fall away. The second stage, which has no wings, will ignite 94 seconds into flight and push IRIS higher and faster into space. The third stage will take over after that, delivering IRIS into its orbit about 10 minutes after launch.

This is the last one scheduled for the Pegasus rocket because there are not any small spacecraft missions that fit the Pegasus niche.

The launch is taking place from the West Coast because IRIS will go into a roughly polar orbit, meaning it will cross over the north and south pole regions of Earth on each pass around the planet.

“Eight months out of the year, we are freely viewing the sun in that orbit,” Hall said.

Once IRIS is in space with its solar panels unfolded to provide electricity and the telescope flipped open, scientists expect to see intriguing data pretty quickly.

“I think the biggest surprise will come once the mission is launched and it starts to observe the sun,” Daw said. “We know to some extent what we hope to learn, what specific science questions we are going to answer, but there’s always that element of surprise.”